Oblong number

Statement

The $k$-th oblong number is $O_k=k(k+1)$ (the dots in a $k\times(k+1)$ rectangle). Compute $O_{10}$ and check it is twice a triangular number.

Solution

$O_{10}=10\cdot 11=\boxed{110}.$ Since $T_k=\dfrac{k(k+1)}{2}$, we have $O_k=2T_k$; here $O_{10}=2\,T_{10}=2\cdot 55=110.$